The phosphoenolpyruvate:glycose phosphotransferase system (PTS), is a system with diverse functions in the bacterial cell, including the uptake and concomitant phosphorylation of its sugar substrates. One such substrate is the disaccharide of chitin, di-N-acetylchitobiose (ChB). In this process, the phosphoryl group is transferred through a chain of proteins from PEP to the sugar: PEP Enzyme I Hpr IIAChb IIBChb ChB. The last step is catalyzed by the membrane protein IICChb . In this sequence, all of the phosphoproteins contain the P linked to His with the exception of IIBChb, where it is linked to a Cys SH group. While much is known about the indvidual proteins of the PTS, there is virtually nothing known about their interactions with one another. The ChB system is ideally suited for determining the thermodynamics of these protein-protein interactions in the PTS. For example, recombinant homogenous E. coli Enzyme I, HPr, IIAChb and IIBChb have been isolated in quantity, as has phospho-IIAChb. Preliminary experiments indicate that the interaction of IIAChb and IIBChb can be monitored calorimetrically. Further studies are required to confirm the magnitude of the measured binding constant of approximately 105 M. In addition, we plan to determine the thermodynamic parameters involved in the presumed reversible phosphate transfer reaction: phospho-IIAChb + IIBChb IIAChb + phospho-IIBChb. Among other valuable information, this experiment should permit us to calculate the energetics of the hydrolysis of the phospho-S-Cys linkage in phospho-IIBChb. No such information is available to date on any of the thiophospho proteins of the PTS.